A negative active material including an active material core; and a polymer layer disposed on a surface of the active material core, wherein the polymer layer includes a third polymer including a cross-linked product of a first polymer and a second polymer, wherein the first polymer is at least one of polyamic acid, polyimide, or a combination thereof, and includes a first functional group; and the second polymer is water-soluble and includes a second functional group, and wherein the first polymer and the second polymer are cross-linked by an ester bond that is formed through at least one reaction starting from the first functional group and the second functional group, and at least one of the first polymer and the second polymer further includes a halogen group.